Accommodation: berths in the hulls#

Sunwake SW-15 · SW-ACC Rev 1 · August 2026

This study explains the decision to place both queen berths in the hulls, the cost of that arrangement, and the geometry showing that they fit. An earlier version assumed that wider sleeping hulls would compromise performance, but the resistance model had not tested beam sensitivity. The revised analysis finds that hull beam has little effect at fixed length and displacement, and the berths fit for a power penalty of one or two percent or less. Companion to the lines plan findings 3 and 4.

1. Beam sensitivity#

Method. Michell's thin-ship wave-resistance integral over the hull, with twin-hull interference at 5.00 m centres, at fixed displacement and fixed waterline length: beam is varied and draft re-solves to hold volume, providing a like-for-like comparison. Friction from ITTC-1957 on each variant's computed wetted girth. Absolutes are calibrated to the powering model; the deltas are the deliverable.

Beam slenderness is not the controlling variable. At fixed length and displacement, widening the waterline from about 1.15 m to 1.30–1.45 m changes bus power by a couple of percent, which is inside model noise, and solar-noon speed by hundredths of a knot. The wave-drag increase from beam is very nearly cancelled by the shallower draft that follows: less volume-depth, less wetted girth, and a small reduction in wetted surface.

The performance case depends primarily on weight, length and form: the prismatic coefficient, LCB, run and transom immersion. Within the range tested, beam is mainly an accommodation variable.

The earlier analysis conflated L/B with volumetric slenderness L/∇^⅓. Volumetric slenderness is set by weight and length, not by waterline beam at fixed displacement, and it is the one that governs. On this boat it is 8.17 and it does not move when the beam does.

Model caveats: thin-ship theory, an interference-phase wobble of a percent or two across the speed range, and parametric sections. Treat the deltas as "indistinguishable", not as "wider is faster". [ENG: CFD confirms at refair.]

2. Queen-berth fit#

The berth is a 2.03 × 1.52 m queen lying outboard between x = 10.40 and 12.43 m, head forward, on a platform 0.70 m above the waterline datum. The hull tapers slightly toward the foot; orienting the berth head-forward makes that unnoticeable.

Measured across the hull at the platform, allowing 55 mm per side for structure and lining, from the faired offsets:

The limiting foot width passes comfortably, and the test suite will catch a future refair that narrows the hull too far.

The under-berth void — at least 1.0 m deep and 1.1 m wide over the berth's length — takes the 64 kWh 352 V island with room for the 96 kWh growth. The battery occupies space already created by the berth platform, low and near the longitudinal centre of gravity. The sealed, insulated and heated trays are removable through platform hatches without demolishing cabinetry.

Headroom follows the brief: standing 2.05 m in the dressing well at the companionway, sitting 0.76–0.84 m over the mattress with the optional trunk coachroof and 0.62 m without. So the trunk lights are an aesthetic choice, not a necessity — which is the right place for that decision to sit, since it is one of the two remaining gates before the general arrangement.

Each sleeping compartment gets a deck hatch for egress and air, and an opening port. The machinery stays aft of an insulated bulkhead at 13.35 m.

3. The layout comparison#

Three arrangements were considered seriously.

C is the chosen arrangement. It adds about 205 kg, reduces solar-noon speed by 0.05 knots and adds two months of build time. In return, the house falls from about 60 % to 46 % of LOA, both cabins have doors, the saloon no longer converts to a bedroom, and the main-deck layout has more room.

The main compromise is access: every berth requires stairs, and the hulls are no longer reserved for machinery and storage. That is an owner preference rather than an engineering decision. The analysis establishes that the performance cost is small.

Every layout keeps the long roof on posts. Shortening the solar array with the house would cost about 2.4 kWp and roughly half a knot of solar-noon speed — far more than any hull-form effect in this study, and more than the whole chine question in the hull form study is worth.

4. Main-deck arrangement#

With both cabins below, the bridgedeck is free to be one room.

Single-level bridgedeck living. Cockpit sole, saloon sole and side decks are all at 1.12 m above the waterline, and the saloon's after wall opens wide to the cockpit, making the after half of the boat one indoor-outdoor space.

• Galley aft to port, facing the cockpit, so the cook faces the view and hands food straight to the outdoor table. Induction; no propane aboard.
• Helm forward to starboard with a chart table, driven from a chair behind real glass with about 300° of visibility. Small wing stations at each after quarter for docking. No flybridge.
• Dinette forward to port, a fixed U-settee around a proper table, convertible for a fifth and sixth berth at anchor.
• Cockpit as the outdoor dining room, under the roof overhang, with the tender on chocks on the after platform.
• Foredeck a solid cambered centreline walkway with netting either side.

Starboard hull: owner's cabin aft, head and separate shower forward — in effect a semi-ensuite suite. Port hull: guest cabin aft, with the forward half given to systems, workshop and stowage. Keeping one forebody clear of accommodation gives every system a serviceable home and protects the weight budget.

Longitudinal arrangement, hull by hull#

Watertight bulkheads at 1.50, 6.00, 9.90, 13.35 and 14.55 m, with crash bulkheads at both ends.

5. Seakeeping and comfort#

Hull separation gives a transverse GM of about 14.5 m, so the natural periods land at roughly 1.7 s in roll, 1.3 s in heave and 1.6 s in pitch. Ocean energy lives at 4–12 s, so there is no resonance overlap at all: the boat contours waves rather than amplifying them.

The snappy motion characteristic of catamarans occurs at 0.6–0.7 Hz, where the ISO 2631 motion-sickness weighting is 0.04–0.08 — roughly twenty times less nauseating than the 0.167 Hz peak where a monohull's roll period typically sits. The motion is likely to feel quick rather than strongly nauseating.

Slamming, not nausea, is the comfort limit, which is why bridgedeck clearance is held at 6 % of waterline length and why the wet-deck panel gets an FEA rather than a rule of thumb.

Features intended to reduce motion sickness include about 300° of horizon reference through the glasshouse, single-level living on the bridgedeck, and an electric drivetrain without exhaust smell, fumes or engine vibration.

Putting the berths below costs a little of the first of those — a sleeping cabin in a hull has a port, not a horizon. In practice that matters at anchor, where the motion is smallest, and the trade was made knowingly.

6. Open items#

1. Trunk lights over the queens, yes or no. An aesthetic gate, not a functional one, and one of the two decisions the general arrangement waits on.
2. The waterline beam refair to 1.45–1.50 m carried in the lines plan finding 2. It would make the berths friendlier still and buy back the Bahamas draft margin at no measurable power cost. If it is taken, re-run the berth check; the test suite will enforce the limit.
3. Ventilation and condensation in the sleeping hulls, which is a Maine April and October problem rather than a summer one. [ENG: dew-point map with the insulation specification.]
4. Stair geometry from the saloon into each hull, which is the real ergonomic cost of arrangement C and has not yet been drawn.